Hydrocarbon resources such as petroleum or natural gas have come to be produced by excavation through wells (oil wells or gas wells, also collectively called “wells”) having a porous and permeable subterranean formation. As energy consumption increases, deeper wells are being drilled, reaching depths greater than 9000 m worldwide and greater than 6000 m in Japan. In wells that are continuously excavated, the productive layer is stimulated in order to continuously recover hydrocarbon resources efficiently from subterranean formations of which permeability has decreased over time and subterranean formations of which permeability has gradually become insufficient. Known stimulation methods include acid treatment and fracturing methods. Acid treatment is a method in which the permeability of the productive layer is increased by injecting a strong acid such as hydrochloric acid or hydrogen fluoride into the productive layer and dissolving the reactive components of bedrock (carbonates, clay minerals, silicates, and the like). However, various problems that accompany the use of strong acids have been identified, and increased costs, including various countermeasures, have also been pointed out. Thus, methods for forming fractures in the productive layer using fluid pressure (also called “fracturing” or “hydraulic fracturing”) have received attention.
Hydraulic fracturing is a method in which fractures are generated in the productive layer by fluid pressure such as water pressure (also simply called “hydraulic pressure” hereinafter). Generally, a vertical hole is drilled, and then the vertical hole is curved and a horizontal hole is drilled in a subterranean formation several thousand meters underground. Fracturing fluid is then fed into these boreholes (meaning holes provided for forming a well, also called “downholes”) at high pressure, and fractures are formed by the hydraulic pressure in the deep subterranean productive layer (layer that produces the hydrocarbon resource such as petroleum or natural gas), and the productive layer is thereby stimulated in order to extract the hydrocarbon resource through the fractures. The efficacy of hydraulic fracturing has also been examined for the development of unconventional resources such as shale oil (oil that matures in shale) and shale gas.
The following method is typically used to perform a well treatment such as producing fractures and perforation by hydraulic pressure using a high-pressure fluid such as fracturing fluid in the productive layer of a deep subterranean formation (layer that produces a hydrocarbon resource such as petroleum such as shale oil or natural gas such as shale gas). Specifically, a prescribed section of a borehole (downhole) drilled in a subterranean formation several thousand meters deep is partially plugged while isolating sequentially from the tip portion of the borehole, and fractures are produced or perforation is performed in the productive layer by feeding a fluid such as fracturing fluid at high pressure into the plugged section or using a tool containing an explosive compound such as a perforation gun. Then, the next prescribed section (typically ahead of the preceding section, i.e., a segment closer to the ground surface) is plugged, and fracturing and the like are performed, causing the fractures and perforations to advance. After that, this process is repeated until the required isolation and fracturing and the like have been completed.
Stimulation of the productive layer is sometimes also performed again by fracturing not only for drilling of new wells but for desired sections of boreholes that have already been formed. In this case as well, the operations of borehole plugging, fracturing, and the like are similarly repeated. Additionally, there are also cases where, to perform finishing of the well, the borehole is plugged to block fluid from below, and after finishing of the top portions thereof is performed, the plugging is released.
A variety of downhole tools, which are tools used in a well to perform plugging and fracturing of a borehole, are known. For example, US 2011/0067889A1, US 2011/0277989 A1, and US 2005/0205266 A1 disclose plugs (also called “frac plugs”, “bridge plugs”, “packers”, and the like) which plug or fix a borehole, having various members (various elements) disposing about the outer surface of a mandrel.
US 2011/0067889 A1 discloses an expandable and degradable plug in which a slip made of a metal or a seal made of an elastomer is disposed on the outer circumferential surface of a mandrel. US 2011/0277989 A1 discloses a degradable downhole plug comprising a slip, a conical member, or a malleable element formed from an elastomer or rubber or the like disposed on the outer circumferential surface of a mandrel, and an impediment such as a ball or flapper. US 2005/0205266 A1discloses a biodegradable downhole tool (frac plug) in which a packer element assembly comprising a slip and a plurality of sealing elements is disposed on the outer circumferential surface of a long tubular body member.
Furthermore, US 2010/0132959 A1 discloses a sleeve system (also called a “frac sleeve”) in which fracture sleeve pistons (also called “pistons” or “piston plugs”), in which a passageway penetrates through the center part, are sequentially arranged so as to be movable in the axial direction of the sleeve, and sequential closed spaces are formed by ball sealers (also simply called “balls”) and ball valve seats (also called “ball seats” or simply “seats”).
Because downhole tools used for well drilling are sequentially disposed in the borehole until the well is completed and well treatments such as fracturing and perforation by high-pressure fluid are performed, they need to have sealing performance such that they plug (seal) the prescribed sections inside the borehole against the fluid pressure. At the same time, the seal needs to be easy to release when any well treatment is finished and the subsequent well treatment is to be performed. Additionally, the seal needs to be released and the used downhole tools removed at the stage when production of petroleum such as shale oil or natural gas such as shale gas (hereinafter collectively called “petroleum and natural gas” or “petroleum or natural gas”) is begun. Because a downhole tool such as a plug is typically not designed to be retrievable after use and release of plugging, it is removed by destruction or by making it into small fragments by milling, drill out, or another method, but substantial cost and time are required for milling, drill out, and the like. There are also plugs specially designed to be retrievable after use (retrievable plugs), but since plugs are placed deep underground, substantial cost and time are required to retrieve all of them.
Additionally, downhole tools used in well drilling are arranged sequentially inside the well until the well is completed, and well treatment such as fracturing and perforation are carried out using high-pressure fluid. Then, various sensors, flow paths, and the like are arranged as downhole tool members in order that all well treatments can be completed, the seal can be released, and the next well treatment can be executed repeatedly in sequence. For these sensors, flow paths, and the like, when downhole tools are arranged inside a subterranean borehole, protection is performed by a protecting member or a protective coating so that breakage or damage does not occur due to friction, due to contact or collision with other members, or due to the high-pressure fluid used in well treatment. For example, a rubber material such a urethane rubber is used. When the sensors or flow paths are to perform their required functions, the protecting member or protective coating needs to be removed. Therefore, it has also come to be demanded that the protecting member for a downhole tool which protects the sensors, flow paths, and the like has a protective function for the sensors, flow paths, and the like, as well as a function of being easily removable or recoverable.
US 2011/0067889 A1 discloses that a slip or mandrel is formed from a degradable metal element such as a reactive metal. US 2011/0277989 A1discloses having flappers, balls, and the like that degrade at a predetermined temperature, pressure, pH, or the like. US 2005/0205266 A1 discloses that a plug or member thereof is formed from a biodegradable material, but does not disclose specific uses. Furthermore, US 2010/0132959 A1 does not disclose that a frac sleeve is degradable.
Due to increased demand for securement of energy resources and environmental protection, particularly as excavation of unconventional resources expands, on one hand, excavation conditions are becoming increasingly harsh, such as increased depth, on the other hand, excavation is advancing under a diversity of excavation conditions, for example, a diversity of environmental conditions such as temperature conditions from less than 60° C. to approximately 200° C. attendant to the diversification of depth. Specifically, the downhole tool member used in downhole tools such as frac plugs, bridge plugs, packers, cement retainers, and sleeve systems (frac sleeves) need to have various properties. These properties include mechanical strength (tensile strength and compressive strength) to allow the member to be transported to a depth of several thousand meters underground, and oil-resistance, water-resistance, and heat-resistance such that mechanical strength and the like are maintained when the members come in contact with the hydrocarbon resource to be recovered in the high-temperature and high-humidity environment of a deep subterranean downhole. Furthermore, a downhole tool member, for example, a seal member for downhole tools (which qualifies as a rubber member for downhole tools) needs to have various characteristics such as seal performance such that it can maintain plugging even against high-pressure hydraulic pressure by sealing fluid between the downhole tool and the inner wall of the borehole, specifically the casing disposed inside the borehole, when the prescribed space of the downhole is plugged for performing perforation or fracturing, and at the same time, it needs to have the characteristic of being releasable when necessary. Additionally, downhole tool members need to have the characteristics of being both easily removable and capable of improving production efficiency by completely releasing the fluid seal within a desired period under the environmental conditions of the well at the stage when the well for well drilling is completed (as described above, there are a diversity of environments such as temperature conditions attendant to diversification of depth). Furthermore, protecting members for downhole tools need to have the characteristic of protecting sensors, flow paths, and the like while the downhole tools are disposed and during well treatment, and being releasable afterward.
Thus, due to the fact that excavation conditions for well drilling have become diverse such as increase depth, there is a need for a degradable seal member for downhole tools that, by reliably sealing fluid between the downhole tools and the casing, makes various well treatments easy to implement in well drilling that requires sealing operations such as perforation and fracturing; and that is also capable of contributing to decreased expense and shortening of processes and contributing to improved production efficiency due to being designable according to desire so that the sealing function can be reliably maintained for a desired period and the seal can be released during a desired period and its removal and a flow path can be assured in diverse downhole environments. Furthermore, there is a need for a protecting member for downhole tools that is similarly capable of contributing to decreased expense and shortening of processes and contributing to improved production efficiency due to having the characteristic of protecting sensors, flow paths, and the like while the downhole tools are disposed and/or well treatment is performed, and being releasable afterward.